As microwave system components shrink in size through increasing levels of integration, there is a need to miniaturize the rf filtering functional blocks as well, such as narrow-band interference rejection filters or mixer sideband selection filters. Unfortunately, the achievable quality (Q) factor of the reactive filter components (e.g., capacitors, inductors, and transmission lines) decreases with size. Also, in applications which require filter tuning through varactor diodes, the finite Q of these devices will introduce losses as well. These factors make it difficult or impossible to implement passive narrow-band, high-selectivity filters in Monolithic Microwave Integrated Circuit (MMIC) form with acceptable performance using cost-effective, standard fabrication processes. However, it is possible to overcome component losses (enhance their Q-factor) by using active devices as negative resistance generators, which can be integrated in MMIC form.
Several narrow-band UHF/microwave active filter circuits have been demonstrated using discrete components, but they have not found widespread use due to several factors. One of the main problems is that these circuits are very sensitive to fluctuations in both active and passive component parameters, due to such factors as variations in fabrication process and changes in bias voltages and in ambient temperatures. Also, prior art enhancement circuits tend to use fairly complex circuit networks to satisfy both rf impedance and DC bias requirements of the active devices.
There are additional problems with conventional apparatus: in applications which require controlled-bandwidth tunable notch filters, circuit parasitics tend to cause undesirable changes in the notch bandwidth as its center-frequency is varied; the rf power handling capability of these circuits is limited, to a much greater degree than for amplifiers, by the size of the active devices needed to compensate for the power lost in the passive components; and in the rejection band of a multistage active filter, the earlier stages of the filter are subject to the greatest power loading which underutilizes later stages and requires higher capacity earlier stages; in dual loop master-slave active filter control systems which control the gain and frequency of the filter the master oscillation reference is loaded down by the loops themselves and is perturbed by the external frequency reference signals introduced to control the frequency of the filter. These perturbations degrade the tracking between the slaved filter and the master oscillator, so that the filter is incorrectly tuned; also it is often desirable to use an external reference signal at a frequency far removed from the filter tuning frequency.